1. Field of the Invention
This invention relates to ignition devices, and more specifically, to electric safety squibs that are insensitive to, and hence, are incapable of being fired by extraneous radio-frequency signals or electric field effects of ordinary intensity that normally pervade the environment.
2. Description of the Prior Art
Electric squibs have long been widely used for many purposes including the firing of explosives for mining, quarrying, demolition, and highway blasting, and for igniting ordnance devices such as flares, explosives, and rockets. A more recent use of electric squibs is for firing inflators for safety air bags in automotive vehicles, combustible gas generants enclosed in a pressure vessel having emerged as a favored means for inflating passive restraints.
Typically, the energizing terminals of electric squibs are connected by parallel rod conductors to the interior of a container or cup containing a pyrotechnic material. In another form, a coaxial type, one of the conductors is tubular in form and the other conductor is a rod centrally positioned therein. The ends of the conductors in the container, in both types of electric squibs, are joined together by a resistance wire or bridge that is designed rapidly to heat the pyrotechnic material to its ignition temperature when energized with sufficient electrical current.
A long standing problem with electric squibs is that their energizing terminals tend to function as antennas that pick up extraneous radio-frequency and electrostatic energy. Radar signals are of particular concern because of their tendency to form transitory peaks of high intensity that are capable of firing the squib. Such firing is caused either by heating the pyrotechnic material of the squib to its flame temperature, by way of contiguous, metallic parts, or by corona discharge or sparking into the pyrotechnic material.
A solution to the problem proposed in the prior art has been twofold. First, there has been disposed between the conductors of the squib, en route to the resistance wire, devices such as ferrite beads, disc capacitors, and resistance headers. These devices are provided to attenuate extraneous radio-frequency electrical energy, in a manner analogous to eddy current dissipation, thereby to diminish it in magnitude prior to reaching the resistance wire and pyrotechnic material. Additionally, one of the conductors is electrically connected to the outer conductive housing or case that is provided for the squib, and the housing is grounded to portions of the device in which the squib is installed. This connection passes off to ground, through a path other than the pyrotechnic material, electrostatic potentials that tend to build up on the resistance wire.
While such an arrangement is satisfactory for many uses of electric squibs, it is not permissible for others. In particular, this solution is unacceptable when the squib is used to fire the inflator for a safety air bag in an automobile, as is disclosed, for example, in U.S. Pat. No. 3,985,076 issued on Oct. 12, 1976, Fred E. Schneiter et al, and assigned to the assignee of the present invention. Specifically, as proposed in the prior art, ferrite beads, disc capacitors and resistance headers have been found ineffective to reduce to an acceptable low level radio frequency energy that reaches the resistance wire and pyrotechnic material. The effectiveness of ferrite beads in attenuating radio-frequency energy has been found to diminish appreciably in the presence of sustained bursts of radio-frequency energy. Such dimunition in attenuating capacity is due to a tendency for the temperature of the beads to rise as a result of the heat generated therein as they dissipate the radio-frequency energy. The rise in temperature has been found to be so great as to appreciably reduce the paramagnetic characteristic of the ferrite beads, and hence, their capacity for attenuating radio-frequency energy.
Additionally, a permanent connection to ground of one of the squib conductors or terminals, as proposed in the prior art, would interfere with the diagnostic or monitoring circuitry required for automobile safety air bag applications, to which circuitry the squib necessarily is connected. The diagnostic circuitry is provided to check for proper operation of the impact sensors and the circuitry associated therewith each time the automobile is started, and a permanent ground connection of a squib terminal would give a false indication of proper operation in the presence, for example, of a short in the cable harness.